Turbochargers for gasoline and diesel internal combustion engines are known devices used in the art for pressurizing or boosting the intake air stream, routed to a combustion chamber of the engine, by using the heat and volumetric flow of exhaust gas exiting the engine. Specifically, the exhaust gas exiting the engine is routed into a turbine housing of a turbocharger in a manner that causes an exhaust gas-driven turbine to spin within the housing. The exhaust gas-driven turbine is mounted onto one end of a shaft that is common to a radial air compressor impeller mounted onto an opposite end of the shaft. Thus, rotary action of the turbine also causes the air compressor impeller to spin within a compressor housing of the turbocharger that is separate from the exhaust housing. The spinning action of the air compressor impeller causes intake air to enter the compressor housing and be pressurized or boosted a desired amount before it is mixed with fuel and combusted within the engine combustion chamber.
The amount by which the intake air is boosted or pressurized is controlled by regulating the amount of exhaust gas that is passed through the turbine housing by a wastegate, and/or by selectively opening or closing an exhaust gas channel or passage to the turbine running through the turbine housing, and/or by adjusting the position of one or more vanes within the turbine housing to vary the gas flow velocity of exhaust gas to the turbine. For example, the use of adjustable vanes within a turbine housing can be used as one way of reducing turbo lag, i.e., the lag time between the time that the vehicle is accelerated from idle and sufficient pressure is developed by the turbocharger compressor to effect an appreciable increase in engine power, by reducing the flow area within the turbine housing to provide the necessary power to quickly accelerate the turbine wheel. As the volumetric flow rate of exhaust gas increases with increasing engine RPM, the vanes are adjusted to increase the flow area within the turbine housing to enable the exhaust gas to generate the appropriate power to compress the necessary quantity of inlet air.
Turbochargers constructed having such an adjustable member within the turbine housing are referred to in industry as variable geometry turbines (VGTs). The movable member within such VGTs, in the form of vanes, nozzles or the like, is positioned in the turbine housing between an exhaust gas inlet or volute and the turbine. The movable member is activatable from outside of the turbine housing by suitable actuating mechanism to increase or decrease the exhaust gas flow within the turbine housing to regulate the air intake boost pressure as called for by the current engine operating conditions, as explained above.
VGTs known in the art can be actuated by using a pneumatic activating means, i.e., by using compressed air or the like or by hydraulic activating means, i.e., by using a pressurized fluid such as oil or the like. An example hydraulically activated actuator includes one comprising a combined piston and rack and pinion assembly. The piston in such actuator assembly is reciprocated within a cylinder by pressurized oil that is passed through a dedicated oil passage within the turbocharger. The oil is passed to the piston at a particular pressure using a valve. A rack and pinion assembly is used with the piston to convert reciprocating piston movement into rotary movement that ultimately actuates the movable member within the turbine, e.g., a VGT vane or nozzle.
A concern with the above-described design is that, due to spatial constraints, the use of a combined piston and rack and pinion assembly requires that the oil passage through the turbocharger be limited in diameter, thereby reducing the response of the actuator assembly to oil pressure. Additionally, the use of such combined piston and rack and pinion assembly requires additional space for proper assembly operation, thereby precluding packaging the assembly in a compact manner to both conserve space around the turbocharger unit and to minimize assembly exposure to radiant heat transfer caused by the intrusion of one or more component to the outline limits of the turbocharger.
It is, therefore, desired that an actuator assembly for a VGT be constructed in a manner that both improves actuator response to an activating means, and improves movable member response to the actuator, i.e., provides a more direct actuator movement to movable member movement. It is desired that such actuator assembly also be constructed having a compact size, when compared to conventional VGT actuators, to both increase available space around the turbocharger and minimize or eliminate exposure to undesirable heat effects.